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Old 11-19-11, 11:52 AM   #5
AC_Hacker
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Quote:
Originally Posted by MN Renovator View Post
Your peak heat load is not on the worse month, it is on the worst part of the coldest night...

The way I calculated the load with my house is I monitored the run time for the 9 hour period where the temperature was in the -20f region last year on January 21st. I basically paid attention to the forecast for the coldest days and logged the on-off of the call for heat which is very close to the time the gas valve is operating and then did the math for the duration it was on divided by the total amount of time I was logging and multiplied that duty cycle percentage against the output rating of my furnace. That day I calculated 27700 BTU/hr would be required....
This is really a great approach, very well explained, too.

I used a method that is similar, in that I live in a small house, and I got enough electric heaters at various thrift stores, to heat the house and bought a Kill-a-Watt for each heater. Then I regularly monitored (4-hr intervals) the electric consumption of the heaters during one of our 99% cold snaps. The period that had the largest summation of energy use was what I used for the max heat load.

I also tried several Manual-J based programs, and found that they came close to predicting what I measured, but the differences calculated by the programs was not a confidence builder.

There seems to be a rule of thumb in the heating trade, to do a Manual-J calculation and then increase the result by 50% when you select the furnace or boiler. In fact, one program I tried had the 50% factor built in.

This 50% increase thing works OK when you use fossil fuel for heating (it certainly reduces 'call-backs'), but for sizing a heat pump it is not so good. Heat pumps are best sized slightly smaller than the maximum, and an axillary fuel is used to fill that gap. That method will result in the greatest efficiency and economy. So, for heat pump sizing, greater heat load precision is called for, and from my experience, the greater precision is had by actual measurement.

Thanks again for you clear approach.

-AC_Hacker
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